Medical imaging, such as X-ray, CT (computed tomography), MR (magnetic resonance imaging), PET (positron emission tomography), and the like have become important clinical tools for evaluation of brain function. One such functional parameter is the cerebral perfusion, which characterizes the passage of blood through the vessels of the brain. Such evaluation procedures are non-invasive or minimally invasive, and may measure the cerebral perfusion by a variety of hemodynamic measurements such as cerebral blood volume (CBV), cerebral blood flow (CBF) and mean transit time (MTT).
The measurement technique may include the administration of contrast agents (which may also be called “tracers”), the tracers being selected as appropriate for the imaging modality. For example, paramagnetic contrast material may be used in MR, and iodinated radiographic contrast material is used for X-ray based modalities.
Using a model of the physiology by which the contrast agent passes through the organ, the hemodynamic parameters may be determined. In the case of the brain, the blood-brain barrier is presumed to restrict the contrast agent to the vascular system and that there is no diffusion in to the extravascular space occupied by the brain tissue. The imaging may be performed as a series of dynamic images during the bolus injection, or after an equilibrium state has been reached.
Dynamic imaging enables visualization and measurement of the first pass of the bolus of contrast agent through the organ to be evaluated. In the case of the brain, the time for a bolus to make a first pass is approximately 20 seconds. A signal representing the concentration of tracer can be evaluated to characterize the hemodynamics.
While the definitions of cerebral blood volume (CBV), cerebral blood flow (CBF), and mean transit time (MTT) may have some variation in the literature, CBV may be considered to refer to the volume of blood in a given region of brain tissue and is measured as milliliters per 100 grams of brain tissue. CBF refers to the volume of blood per unit time passing through a given region of brain tissue, and is measured in milliliters per minute per 100 grams of brain tissue; and, MTT refers to the average time taken for blood to pass through a given region of brain tissue, and is measured in seconds. Other characterizing measurements may also be performed.
In the clinical practice, the tracer or contrast agent is usually injected manually during an angiographic examination. Only in few cases, e.g., for specific clinical studies where the exact protocol of injection is required, will automatic injection with a power injector be utilized. In the routine clinical case, usually only the total dose of the tracer injected during the whole angiographic procedure will be recorded. The doses of the tracer and the injection rate for an individual image data set acquisition may not have been recorded.
A common method of analyzing the images is to measure the tracer intensity profile in the main feeding artery as representative of the input function. In the case of an artery of the brain, this is termed the arterial input function (AIF). This analysis is typically performed by manually selecting a portion of the image representing a region of the feeding artery and extracting the time-concentration-curve of the tracer at this region. This may be aided by narrowing the selection to a small group of pixels chosen using an automated algorithm which searches the entire image data for pixels with time-concentration curves that satisfy criteria characteristics of arteries, such as large peak, early arrival time, and short mean transit time. This may require the manual establishment of time and intensity thresholds.